As a conventional induction heating cooker, there is known an induction heating cooker recited in e.g. patent document 1. FIG. 31 is a diagram showing an arrangement of the conventional induction heating cooker. As shown in FIG. 31, the conventional induction heating cooker is constructed in such a manner that a vibration detecting section 114 is provided at a position underneath a top plate 112, on which a cooking vessel 111 is to be placed, to detect a vibration resulting from generation, detachment, disappearance and the like of air bubbles which may occur by heating an induction heating coil 113. A vibration amplifying section 115 detects a vibration of the cooking vessel 111 via the top plate 112. Upon detecting generation of air bubbles resulting from boiling, a boiling determining section 116 detects whether an object to be heated has been boiled, and a heating controlling section 117 reduces the output of the induction heating coil 113 to avoid overflow of air bubbles and the like.
A conventional approach for detecting a temperature of the top plate 112 by a thermistor or a like device is directed to improve degradation of responsiveness resulting from a low thermal conductivity of the top plate 112 or the like. FIG. 32 is a graph showing a time-based change in vibration waveform of the induction heating cooker recited in patent document 1. As shown in FIG. 32, the temperature of an object to be heated i.e. water is increased by a high-frequency induction heating at about 20 kHz. As the water temperature is increased, partial boiling occurs from a bottom surface of the cooking vessel 111, and a vibration at a relatively low frequency of 10 kHz or less is sharply increased. Then, as the water temperature approaches the boiling point, air bubbles grow into large air bubbles, and a vibration is slightly reduced. After the object to be heated has been boiled, the magnitude of vibration is settled to a certain level. The boiling determining section 116 of the conventional induction heating cooker detects whether the object to be heated has been boiled, based on the above change in the vibration waveform.
In the case where a cooking vessel 111 is fluorine coated, an air bubble generating condition is different from the above case. Accordingly, a time-based change in vibration of the cooking vessel 111 is different from the above case. FIG. 33 is a graph showing a time-based change in vibration waveform of an induction heating cooker recited in patent document 2. As shown in FIG. 33, in an initial stage of air bubble generation, the induction heating cooker 111 is vibrated at a low frequency of 10 kHz or less, at which the vibration is relatively easily transmitted. However, since air bubbles grow from a bottom part of the cooking vessel 111, a vibration resulting from detachment or disappearance of air bubbles is less likely to be detected. By the time when the size of the air bubbles is sufficiently increased, and the large air bubbles are detached from the bottom part of the cooking vessel 111, the temperature of the entirety of the object to be heated is also sufficiently increased. At this time, the air bubbles reach a top surface of the object to be heated, and are released into the air, without disappearing before reaching the top surface of the object to be heated. A boiling determining section 116 determines whether the object to be heated has been boiled, based on a change in the vibration waveform output (see e.g. patent document 2).
In patent document 1, the point of time T2, at which it is determined that the object to be heated has been boiled, is delayed from the point of time T1 corresponding to the boiling point 911 shown in FIG. 32. This may be inverse in time depending on the kind of the cooking vessel 111. Also, in the case where the object to be heated includes a material other than water, and a solid matter such as a foodstuff is put into the cooking vessel 111 in the course of cooking, it is impossible to determine whether the foodstuff has been put in before air bubble generation, although it is possible to detect lowering of air bubble generation resulting from temperature lowering if the air bubbles have already been generated.
In particular, patent document 2 recites a method which is also feasible in the case where a fluorine-coated cooking vessel is used. However, in recent years, a product formed with an aluminum alloy plate layer having a thickness of 4 to 5 mm or more on or around the center on a bottom surface of a cooking vessel 111 has been increasingly used to enhance thermal efficiency by induction heating. Therefore, it is substantially difficult to detect a vibration corresponding to detachment of large air bubbles or burst of air bubbles at a top part of the object to be heated, and it is impossible to avoid overflow of air bubbles and the like by detecting whether the object to be heated has been boiled. In other words, it is less likely to obtain the output voltage as shown in FIG. 33.
As another drawback, in the case where a cooking vessel containing an object to be heated is heated with use of an induction heating cooker, the cooking vessel may be resonated with a vibration of the induction heating cooker, with the result that a resonant sound may be generated. In the conventional induction heating cooker, it is difficult to detect whether a resonant sound has been generated, and it is difficult to suppress generation of a resonant sound.    Patent document 1: Japanese Unexamined Patent Publication No. Sho 62-243282    Patent document 2: Japanese Unexamined Patent Publication No. 2003-77643